When the Earth started life was pretty dull consisting of algae, plankton and bacteria. But then, about 600 million years ago, life exploded; evolving into some of the weirdest creatures the world has ever seen. Looking at some of these creatures that lived in this age it is bizarre to think that they evolved into animals that would one day be us…
This was a group of small horse-shoe crab-like arthropods. It used to be thought that these were the ancient relatives of modern day horse-shoe crabs; however there is much debate over this. Now, it is believed that they are related closely to trilobites. Fossils of these creatures are found all over North America, Europe and Australia.
This creature literally means ‘Terror Crab’. They were most probably the apex predators of their time. They are bilaterally symmetrical with a non-mineralised cuticle and two body segments. The frontal section contains claws in the mouth area. Swimming lobes are thought to allow this creature to swim through the water like a cuttlefish. Its fossils are found in China, Russia and Canada.
These strange creatures bodies comprised of two parts; the bulbous front containing the mouth and five oval to round shaped ‘gills’ and the tail-like posterior comprising of seven segments. They seem to lack any preserved appendages. There is much dispute as to what these prehistoric animals actually are. Many people believe them to be crown and stem-group arthropods, while others believe they are stem-group vertebrates or even early deuterostomes. It is assumed that they were fully living organisms, spending most of their lives in the water column. None of the mouth parts discovered had any features for chewing, so it is thought that they were not predators. To add to this, their stomach contents contained sediment infill, suggesting that they were filter feeders. However, some scientists think they were ‘selective deposit-feeders’.
These are commonly known as ‘comb jellies’ and live in waters worldwide. They use ‘combs’ (groups of cilia) for swimming. They consist of a mass of jelly and have a nerve net rather than a brain. Water flows through the body cavity for both feeding and respiration. Oddly enough, comb jellies are predators, feeding on larvae and small crustaceans and can be greedy, sometimes eating up to ten times their weight in one day! They have no hard parts, but excellently preserved fossils have been found in Cambrian lagerstätten, such as the Burgess Shale, all lacking tentacles but containing 24-80 comb rows (even though modern comb jellies have 8 rows). Oddly enough, fossil ctenophores seem to have internal organ-like structures, unlike anything we see in modern representatives.
So why did the explosion happen?
When we think of the word ‘explosion’, we think of a sudden burst. And in geological terms, yes, life did ‘explode’ very rapidly, but it still happened over many millions of years. All major body plans came out of this explosion, including the beginning of the Phylum Chordata and Mollusca. The answer to the explosion may be explained by looking at a major geological mystery: The Great Unconformity. This shows an unconformity between igneous and metamorphic rock in the Earths crust and relatively younger Cambrian sedimentary marine rocks. The cause of The Great Unconformity may also have been the catalyst for the Cambrian Explosion. Peters states:
“Our hypothesis is that biomineralisation evolved as a biogeochemical response to an increased influx of continental weathering products during the last stages in the formation of the Great Unconformity.”
Shallow seas in the Cambrian went through peaks and troughs, eroding away at the uncovered rocks and exposing the crust rock, which reacted with the air and water. This released iron, calcium, potassium and silica into the sea- some of which are great for building shells.
The appearance of new animals led to the new animal interaction, predation. This allowed ecosystems to become much more complex, as animals needed to adapt to survive. This meant that organisms filled almost every environmental niche, such as benthic and nektonic lifestyles. It also allowed them to grow to various shapes and sizes. In particular, there was a great boom in the shapes and sizes of small shelly fossils, such as gastropods. Below is an image that shows the size of different Cambrian animals.
Trilobites are definitely a success story of the Cambrian; they survived almost 300 million years and could live in all marine settings, such as burrowed in the sediment, in the plankton, as free swimmers or in burrowed caves. Their morphologies were also majorly diverse; many had spines, head ornamentation and some had complex eyes or no eyes at all! (Suggesting life in dark environments)
To add to this, trace fossils became more complex during the Cambrian Explosion. Before this, only simple horizontal surface traces existed, but animals began to dig vertically into the sea floor, suggesting that new appendages, that were able to dig, evolved.
Hinde, R.T., (1998). “The Cnidaria and Ctenophora”. In Anderson, D.T.,. Invertebrate Zoology. Oxford University Press. pp. 28–57
Ortega-Hernández, J.; Braddy, S. J.; Jago, J. B.; Baillie, P. W. (2010). “A new aglaspidid arthropod from the Upper Cambrian of Tasmania”. Palaeontology 53 (5): 1065–1076
Usami, Y. (2006). “Theoretical study on the body form and swimming pattern of Anomalocaris based on hydrodynamic simulation”. Journal of Theoretical Biology 238 (1): 11–17
Vinther, J.; Smith, M. P.; Harper, D. A. T. (2011). “Vetulicolians from the Lower Cambrian Sirius Passet Lagerstätte, North Greenland, and the polarity of morphological characters in basal deuterostomes”. Palaeontology 54